I should note, there is a WebmasterWorld thread with complaints about his ad prices soaring recently. He said:

One by one this change is sweeping through my various AdCenter accounts... last month there was a change made to the broad matching algorithm that loosened match criteria. In our case, the volume of impressions shot up by 25% in two days and our CPA went straight to hell because the new traffic is irrelevant, untargeted bunk. I know from friends in other companies that we're not the only ones having this problem. Frankly I think a rollback and a refund are in order but who listens to me lol.